The orbital anisotropy profiles of nearby globular clusters from Gaia Data Release 2

Abstract

Gaia Data Release 2 provides a wealth of data to study the internal structure of nearby globular clusters (GCs). We use this data to investigate the internal kinematics of 10 nearby GCs, with a particular focus on their poorly studied outer regions. We apply a strict set of selection criteria to remove contaminating sources and create pure cluster-member samples over a significant fraction of the radial range of each cluster. We confirm previous measurements of rotation (or a lack thereof) in the inner regions of several clusters, while extending the detection of rotation well beyond where it was previously measured and finding a steady decrease in rotation with radius. We also determine the orbital anisotropy profile and determine that clusters have isotropic cores, are radially anisotropic out to ≈4 half-light radii or 35{{ per cent}} of their limiting radii, and are then isotropic out to the limits of our data sets. We detect for the first time the presence of radial anisotropy in M 22, while confirming previous detections of radial anisotropy in 47 Tuc, M 13, M 15, and ω Cen's innermost regions. The implications of these measurements are that clusters can be separated into two categories: (1) clusters with observed radial anisotropy that likely formed tidally underfilling or are dynamically young, and (2) clusters that are primarily isotropic that likely formed tidally filling or are dynamically old.